Should a fault occur in a liquid dielectric distribution transformer, there is a risk of the dielectric heating up and causing pressure rise to a level such that an explosion could occur, leading to serious consequences to the environment caused by the dielectric being sprayed about.
French patent No. 2 629 955 describes a device for protecting a three-phase transformer immersed in a liquid dielectric from a fault consisting in the dielectric liquid dropping to a predetermined level.
The device consists in that each phase is equipped, on its high voltage side, with a current-limiting fuse, and in that if the level of the dielectric drops, a float triggers an outright short circuit between a fourth contact connected to the mass of the transformer and a point in each phase situated between the fuses and the windings of the transformer.
However, in a neutral-compensating network, as was mentioned above, such a device is not satisfactory. Indeed, in the event of a fault (a drop in level such that the float establishes contact between the four points) there will be a three-phase short circuit connected to ground via the contact to mass, and only two of the fuses will blow with electricity continuing to pass through ground and the third phase which has not blown, at a current that is too low to operate the third fuse, but that is high enough either to trip the source station or else to heat the liquid dielectric, thus causing the pressure to rise and the tank to explode.